In the disk subsystem that has one or more hard disks and a disk controller that controls the hard disk and transfers data between the disk controller and a host computer, when the data that is input and output from the host computer to the disk controller is distributed and stored using the RAID architecture, particularly the RAID5 architecture, the number of real data items (D) per parity data (P), that is, the number of drives in a magnetic-file stripe can be determined optionally.
If two hard disks that belong to the same stripe are provided in a fiber loop, however, the two hard disks will not be able to be used at the same time when a fiber loop fault occurred, thereby disabling the recovery of fault data using parity data. Accordingly, to ensure the redundancy of the hard disk when the fiber loop fault occurred, the number of hard disks per fiber loop must be set to 1. Thus, data will be distributed and stored as (n−1) real data items and 1 (one) parity data item for the number of fiber loops (n). A hard disk control unit is formed by collecting m disk adapters that control the fiber loop.
A disk controller realizes scalability by enabling increased and decreased installation in a unit of this hard disk control unit. When there has 1 hard disk control unit in the disk controller, the number of fiber loops is m, thereby establishing n=m. For example, when the number of disk adapters (m) in the hard disk control unit is 4, the number of fiber loops used is set to 4 and the magnetic-file stripe has the format of 3D+1P. Further, when the number of hard disk control units in the disk controller is also 2 or more, the hard disk control unit is operated for n=m by using the same logic as a single hard disk control unit. By distributing and storing data in the hard disk control unit in this manner, the operation of each hard disk control unit is let to have independence and the increased and decreased installation of the hard disk control unit was enabled without affecting the hard disk control unit in course of system operation.